build: store SOURCE_DATE_EPOCH in JSON info files

The source date epoch is the only reproducible date close to the actual
build date. It can be used for tooling like the firmware wizard to show
the image age.

Signed-off-by: Paul Spooren <mail@aparcar.org>
(cherry picked from commit 165f0b00cd)
[store source_date_epoch as integer]
Signed-off-by: Paul Spooren <mail@aparcar.org>

include/image.mk

@@ -532,6 +532,7 @@ define Device/Build/image
 	@mkdir -p $$(shell dirname $$@)
 	DEVICE_ID="$(DEVICE_NAME)" \
 	BIN_DIR="$(BIN_DIR)" \
+	SOURCE_DATE_EPOCH=$(SOURCE_DATE_EPOCH) \
 	IMAGE_NAME="$(IMAGE_NAME)" \
 	IMAGE_TYPE=$(word 1,$(subst ., ,$(2))) \
 	IMAGE_FILESYSTEM="$(1)" \

scripts/json_add_image_info.py

@@ -31,6 +31,7 @@ image_info = {
     "target": "{}/{}".format(getenv("TARGET"), getenv("SUBTARGET")),
     "version_code": getenv("VERSION_CODE"),
     "version_number": getenv("VERSION_NUMBER"),
+    "source_date_epoch": int(getenv("SOURCE_DATE_EPOCH")),
     "profiles": {
         device_id: {
             "image_prefix": getenv("IMAGE_PREFIX"),